A056723 Numbers k such that 8*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 7, 23, 29, 133, 173, 367, 1925, 3707, 5765, 9709, 19573, 43753

Offset

1,2

Comments

Also numbers k such that (25*10^k - 1)/3 is prime.

Links

Table of n, a(n) for n=1..13.

Makoto Kamada, Prime numbers of the form 833...33.

Index entries for primes involving repunits

Mathematica

Do[ If[ PrimeQ[ 8*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 10000}]

Crossrefs

Cf. A002275, A093676.

Sequence in context: A095314 A038948 A019389 * A080802 A078515 A225822

Adjacent sequences: A056720 A056721 A056722 * A056724 A056725 A056726

Keyword

nonn

Author

Robert G. Wilson v, Aug 11 2000

Extensions

Corrected by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

19573 and 43753 from Serge Batalov, Jan 2009 confirmed as next terms by Ray Chandler, Feb 13 2012

Status

approved